The present invention can be understood by considering the limitations of techniques currently employed in locating and identifying concealed objects such as mines, ordnance, weapons, machinery, pipes, cables, tunnels, barrels, hazardous chemicals and pollutants, illicit materials, pests, biological disorders and anomalies, etc., when such concealed objects are located within an obscuring medium (i.e. a medium which is capable of scattering and absorbing radiation and/or distorting the propagation of radiation through variations or inhomogeneities in the local refractive index).
Variations in the refractive index of a medium are encountered in fields such as astronomy (ground based telescopes peering through atmospheric refractive turbulence (which has led to the development of an artificial laser-based guide star, a particular kind of reference object which approximates a point source of radiation)), geology (stratified layers of soil or sea bottom, pools of gas, oil, water, etc.), and medical imaging (ultrasound, optical, microwave). Implementations of this invention are preferably used with obscuring mediums such as snow and ice, gravel, sand, soil, mud, river bottom, lake bottom, ocean bottom, biological waste, sewage or sludge, and human tissue.
The obscuring medium may simply be referred to as the medium. See, Underground and Obscure Object Imaging and Detection, SPIE vol. 1942 (N. Del Grande, et. al., eds. 1993); and Aerial Surveillance Sensing Including Obscured and Underground Object Detection, SPIE vol. 2217 (I. Cindrich, et. al., eds., 1994). In the case of buried mines, chemical or biological sensors (typically sniffer dogs, laser spectroscopy, mass spectroscopy, etc.) have been used with limited success. A number of imaging techniques have been implemented which employ acoustics (seismic studies), ionizing radiation (primarily gamma rays, x-rays, neutrons, and to a lesser extent charged particles), low-energy electromagnetic radiation (mainly covering UV, visible, infrared, microwave, and radar), magnetic fields (such as gradiometers), thermal radiation, etc. Low-energy electromagnetic radiation will be referred to as electromagnetic or optical radiation. Neutrons need not interact with materials solely through ionization to result in radiation (or byproducts) which can be detected. Neutron radiation used in this manner will still be included in the group referred to as ionizing radiation.
Current technologies that employ ionizing radiation may be unacceptable in some circumstances because they may require some type of accelerator (including x-ray tubes) with a heavily filtered output or an intense radioisotope source (since the source itself is self-attenuating, does not produce a directional beam, and radiation of a narrow bandwidth is desirable), and energy-sensitive receivers. In addition, current technologies require significant shielding. Technologies that employ x-ray tubes may suffer from very high power consumption, requiring specialized tubes or instead resulting in a reduction in operational lifetime. It will be shown that our invention will overcome the limitations of these old technologies by filtering and focusing ionizing radiation in order to increase beam intensity, improve beam directional and spectral properties, and/or enlarge the acceptable beam cross section which will, in turn, enable radioisotopes, x-ray tubes, neutron generators, neutron-emitting isotopes, etc., ionizing radiation sources for imaging objects within an obscuring medium.
A number of factors will limit the practical implementation of an imaging technique or system. Scattering, absorbing, and distorting properties of the obscuring medium and object materials typically impose limitations on the waveform and type of radiation. Coupling inefficiencies or losses can result from source and receiver limitations and interfere with the goal of delivering useful radiation from the source to the medium and collecting useful radiation exiting the medium. Additional inefficiencies or losses can result from radiation propagation problems experienced at the interfaces between mediums containing the source, receiver, and object due to discontinuities. Two other conditions, if present, will further degrade current imaging techniques: the presence of additional objects (in addition to any nonuniformity in the local refractive index) and a nonuniform interface (an irregular or rough surface) between the obscuring medium and the mediums containing the source and receiver. For example, an obscuring medium such as the ground can contain stones, roots, voids, etc., and vary in soil composition, moisture content, gases, biology, etc. The surface of the ground can be broken, rough, sloped, etc. These two conditions also complicate optical imaging in tissue such as breast tissue. The viability of a particular imaging technique can be increased if at least some of the effects of coupling inefficiency, a nonuniform surface, and an inhomogeneous medium can be reduced.
Adequate levels of (useful) radiation need to be coupled (focused and directed) efficiently from the source to the region of interest and from the region of interest to a detector. Discontinuities which result from physical properties of the mediums containing the source, concealed object, and detector as well as rough or irregular interfaces must be considered. In some cases external structures such as ground cover (vegetation, etc.) or the effects of wind, temperature, noise (man-made, etc.), and other environmental conditions can introduce additional levels of complexity. Furthermore, the concealed object, such as a land mine, often represents a hazard and is designed and placed to avoid detection via a particular technology. For example, non-magnetic land mines are widely used and many mines are of a shape and size that they are not easily detected using ground-penetrating radar.
The probability of detecting the concealed object can be improved by examining those aspects of the particular problem which we can reasonably hope to influence. The approach taken here is largely based on previous work by the present inventors, Nelson et. al., for medical imaging and specific non-destructive testing problems. See U.S. Pat. No. 4,937,453 (Jun. 26, 1990), U.S. Pat. No. 5,017,782 (Nov. 19, 1990), U.S. Pat. No. 4,958,368 (Sep. 18, 1990), U.S. Pat. No. 4,969,175 (Nov. 6, 1990), U.S. Pat. No. 4,649,275 (Mar. 10, 1987), U.S. Pat. No. 4,767,928 (Aug. 30, 1988), U.S. Pat. No. 4,829,184 (May 9, 1989), U.S. Pat. No. 4,948,974 (Aug. 14, 1990), U.S. patent application Ser. No. 08/480,760, filed Jun. 6, 1995, and U.S. patent application Ser. No. 08/597/447, filed Feb. 2, 1996, the disclosures of which are hereby incorporated by reference as if fully set forth herein. These earlier works describe not only a number of data acquisition and noise correction techniques, but also the advantages of creating an efficient imaging environment with predictable properties.
The identification and classification problems that the present invention addresses require improvements and new additions to the present state of the art. In addition, the various environments (sand, mud, snow, dirt, flooded fields, rough terrain, ground cover, ocean bottom, river bottom, lake bottom, etc.) in which specific implementations of the invention can be utilized imply that requirements such as survivability, low cost, and maintainability should be considered in the design of imaging systems for use in these environments. Survivability problems will be of reduced importance for implementations of the invention which may be employed in other environments (medical, non-destructive testing, etc.).
The ability of an imaging system to locate and identify a concealed object in an obscuring medium can be improved by incorporating enhancements which would: (1) optimize source/receiver parameters, source utilization, and the use of appropriate data acquisition and signal processing techniques and algorithms; (2) optimize radiation transport into, through, and out of the medium; and (3) exploit or modify properties of the object and medium.
Optimizing source/receiver parameters, source utilization, and the use of appropriate data acquisition and signal processing techniques and algorithms, may also involve a mix of imaging technologies (for example, optical and acoustic) and timely acquisition rates. High resolution imaging techniques might be very time consuming in comparison to techniques which are adequate for identifying the presence of potential objects of interest. In addition, the cost of a large area, high resolution detection system may be substantial relative to a small area, high resolution detection system. In many situations it may be more cost-effective to use adequate imaging resolution to detect the presence of potential objects of interest and then use high resolution imaging techniques to help identify the object. Searching a large area using a high resolution imaging method may simply be too time consuming, particularly if the density of potential objects is low.
The radiation field entering and leaving the obscuring medium can be severely modified by scattering, absorbing, and refractive effects. The surface of an obscuring medium such as soil is typically rough and irregular. Vegetation may also cover at least part of the surface. The volume of this medium is likely to be inhomogeneous. Non-invasive detection systems currently in use for applications such as buried mine detection (for example, radar) typically suffer from the radiation transport problems listed. Radiation transport problems may also reduce the incentives to acquire a more complete data set. Often only a single view of the medium is acquired (or at least the range of views is quite limited). Enhancing radiation transport into, through, and out of an obscuring medium such as soil may involve modifying the obscuring medium, its surface, or both. In addition, the likelihood of detecting a concealed object may be further improved by modifying the concealed object or its immediate environment.
New non-invasive and invasive devices and/or modes of imaging an object concealed in an obscuring medium using ionizing radiation, optical, acousto-optical, acoustic (including photo-acoustic) and/or mechanical imaging techniques are needed to improve image quality (including material characterization accuracy). Particular problems which need to be addressed are the need for improved radiation coupling into and out of the scattering medium as well as improved transmission through the medium, the need to enhance the information content of detected radiation (which can involve modifying the object and the medium), and the need to acquire a data set which is more complete (which frequently involves sampling the medium from more than one direction (multiple projections), more than one wavelength or energy, more than one waveform, etc.).
It is desirable to exploit various physical properties of the object and the medium such as reflectivity, scattering, absorption, frequency-dependence, polarization-dependence, material composition, density, internal and external structure, impedance, permitivity, conductivity, emissivity, inductance, dielectric constant, bulk modulus, radiation velocity, radiation cross section, characteristic radiation, resistance to penetration, etc. It is desirable to implement methods utilizing improvements such as increased source coupling efficiency (source utilization efficiency) to the region of interest and a radiation beam with well defined properties (angular distribution, energy distribution, polarization, pulse width, modulation frequency, etc.) to enhance imaging of a concealed object in an obscuring medium when ionizing radiation is employed. For example, it would be desirable to have methods using thermal energy or an acoustic field to modify the properties of a localized volume of the obscuring medium for optical radiation (under the appropriate conditions) also exploited for ionizing radiation.
However, prior devices and methods do not address these concerns.